Rotational seismology is an emerging area for studying all aspects of rotational ground motions induced by earthquakes, explosions and ambient vibrations. This particular field of seismic study has found application to a wide range of geophysical disciplines, including strong-motion seismology, broadband seismology, earthquake engineering and physics, seismic instrumentation, seismic hazards, seismotectonics, and geodesy. Although translational ground motions have been traditionally detected and observed in seismology, scientists have found that measuring the three components of rotational motion and the six or more components of strain will improve our understanding of the earthquake process and the complex ground motions generated by such seismic activity.
In seismology, the general motion of particles or a small volume in a solid body can be divided into three parts: translation (along the x, y, and z axes), rotation (about the x, y, and z axes), and strain. Because it has been found that the recurrence interval of a large earthquake at a given fault line has been approximately between 100 to 10,000 years, seismologists have been trying to optimize their observations for either studying either large earthquakes at great distances or smaller local earthquakes at closer distances. As such, traditional seismographs have been designed to have a high sensitivity at the expense of being able to record large motions on scale.
Until recently, observational seismology has been based mainly on measuring translational motions since rotational motions imparted by seismic activity have been considered insignificant to the seismological observations of earthquakes. Although translational motions induced by seismic activity along the three axes have been observed extensively, yielding essentially all the information we currently know about earthquakes and the structure of the Earth, the rotation about these axes has proven to be very difficult to detect and record due to the lack of suitable instruments. However, during the past few years the pioneering efforts of scientists observing the Sagnac effect caused by rotations on ring laser gyroscopes have started to yield important new results in seismology related to observing rotational motions imparted by seismic activity. Unfortunately, a notable disadvantage of such laser gyroscopes is their prohibitive cost, requirement for sophisticated facilities, and complicated operation.
Accordingly, there is a need for a rotational seismometer that detects and records rotational motions associated with seismic activity in near-field earthquakes that is relatively simple to operate, robust in construction, and affordable with sensitivities to seismic activity that meet the standards set forth by the International Working Group on Rotational Seismology.